1. Field of the Invention
This invention relates generally to earth-boring drill bits, and particularly to improved cutting structures therefor.
2. Background Information
The success of rotary drilling enabled the discovery of deep oil and gas reservoirs. The rotary rock bit was an important invention that made the success of rotary drilling possible. Only soft earthen formations could be commercially penetrated with the earlier drag bit, but the two cone rock bit, invented by Howard R. Hughes, U.S. Pat. No. 930,759, drilled the hard caprock at the Spindletop Field, near Beaumont, Tex., with relative ease. That venerable invention, within the first decade of this century, could drill a scant fraction of the depth and speed of the modern rotary rock bit. If the original Hughes bit drilled for hours, the modern bit drills for days. Modern bits sometimes drill for thousands of feet instead of merely a few feet. Many advances have contributed to the impressive improvement of rotary rock bits.
In drilling boreholes in earth formations by rotary method, rotary rock bits fitted with one, two, or three rolling cutters, rotatably mounted thereon, are employed. The bit is secured to the lower end of a drill string that is rotated from the surface or by downhole motors or turbines. The cutters mounted on the bit roll upon the bottom of the borehole as the drill string is rotated, thereby engaging and disintegrating the formation material to be removed. The roller cutters are provided with teeth that are forced to penetrate and gage the bottom of the borehole by weight from the drill string.
The cuttings from the bottom and sides of the well are washed away by drilling fluid that is pumped down from the surface through the hollow, rotating drill string, and are carried in suspension in the drilling fluid to the surface. The form and location of the teeth upon the cutters have been found to be extremely important to the successful operation of the bit. Certain aspects of the design of the cutters becomes particularly important if the bit is to penetrate deeply into a formation to effectively strain and induce failure in more plastically behaving rock formations such as shales, siltstones, and chalks.
In drilling shales and siltstones, which are the dominant lithologies in oil well drilling, and other earthen formations, two problems frequently arise. One problem, known as "tracking," occurs when the teeth of a cutter fall in the same indentation that was made on the previous revolution of the bit. When this occurs, the teeth of the cutters on the bit are said to "track." Tracking causes the formation of large hills and valleys, known as "rock teeth," on the bottom of the borehole. Rock teeth present a sculptured drilling surface that closely matches the pattern of the teeth of the cutters, making it more difficult for the teeth to reach the virgin rock at the bottom of the valleys. Rock teeth also tend to redistribute the weight on the bit from the crests to the flanks of the cutting teeth, which impedes deep penetration and leads to inefficient material fragmentation, and often to damage to the bit and bit bearings.
The other problem frequently encountered in drilling shales, and other soft earthen formations, is known as "balling." Balling occurs when formation material becomes lodged between the teeth on the cutter of the bit. Balling, like tracking, prevents the teeth of the cutter from penetrating to full depth, thus resulting in inefficient and costly drilling. Balling also prevents the force on the crests of the teeth from reaching a level sufficient to fracture rock.
The characteristics of both tracking and balling are well-recognized, but generally are treated as independent problems. However, in many cases, features that reduce tracking promote balling, and vice-versa. For example, a conventional steel-toothed bit as disclosed in U.S. Pat. No. 1,896,251, Feb. 7, 1933, to Scott, has closely spaced teeth with axial crests intermittently interrupted by wide spaces to reduce the height of rock teeth and the severity of the tracking. Balling is more likely to occur between closely spaced teeth and such teeth are severely limited in their ability to penetrate the formation deeply. U.S. Pat. No. 2,333,746, Nov. 9, 1943, to Scott et al., on the other hand, discloses large and widely spaced teeth with axial crests to minimize balling, particularly in the outermost rows of the earth-boring bit. Such large and widely spaced teeth, however, are prone to tracking and the resulting build up of rock teeth.
There it is a need, therefore, to provide an earth-boring bit having a cutting structure designed to penetrate relatively soft earthen formations rapidly by simultaneously minimizing the occurrence of both tracking and balling.